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I. Kühn and S. Klotz
Similarly, environmental heterogeneity was able to account for species
richness of natives and aliens in the USA (Stohlgren et al. 2006).Although this
pattern of positive correlation between native and exotic species was also
observed at a global scale (Lonsdale 1999), we are not aware of any analysis
demonstrating a set of common drivers behind this relationship. Nevertheless,
it is very likely that the same variables, i.e. energy (temperature) and
water availability (Francis and Currie 2003), are able to at least largely explain
this pattern for both alien and native plant species.
11.6 Scale-Dependent Consequences for Biodiversity
of Invaded Ecosystems
We showed that patterns of ecosystem invasibility changed with spatial scale,
especially resolution.What will the consequences of this be for biodiversity? It
seems short-sighted to focus simply on biodiversity and disregard other welldocumented
impacts of biological invasions involving nutrient cycling (especially
by nitrogen fixers such as Acacia or Myrica faya, the fayatree, Chap. 10),
water table depletion (Acacia or Tamarix ramosissima, the salt cedar), alteration
of soil structure through salt accumulation (Mesembryanthemum crystallinum,
the ice plant) or soil perturbation by digging (the feral pig Sus scrofa
domestica), which additionally disperse seeds of alien plant species and fertilise
the soil (Williamson 1996). However, it is biodiversity or rather, its elements
(i.e. species) which largely drive ecosystem processes. Still, most conservation
actions are concerned with species as such, not with the goods and
services they provide as integral parts of an ecosystem.
We discussed several studies showing an increase of alien species at
higher native species levels at larger spatial scales. At the global scale, however,
alien species are considered to be among the major causes of species
extinctions (e.g. Diamond 1989; Sala et al. 2000; Chaps. 13, 15, 16). This
impact seems inevitable, given that global extinction rates cannot be compensated
by speciation rates. At a global scale, the introduction of species
into a new habitat or biogeographical region does not add to biodiversity but
the loss of a single species due to this introduction decreases biodiversity.
Within regions (i.e. areas which are intermediate in size between those of the
globe and small study plots), Sax and Gaines (2003) show for a variety of
groups of organisms and across many different parts of the world that the
net gain of species due to biological invasions is higher than the loss of
species. As an example from Europe, the German Red List of endangered
vascular plant species (Korneck et al. 1996) lists 47 taxa as extinct and 118
as threatened. On the other hand, 470 vascular plant species are considered
to be naturalised aliens (neophytes; Klotz et al. 2002), and alien species are
not among the major causes for species extinctions in Germany (Korneck et